1. Field of the Invention
The invention relates to a rechargeable electrochemical battery having a solid electrolyte and a thin film cathode and anode. In particular, the battery employs a proton-conducting organic polymer which is a mixture of a strong acid and a base polymer, a thin film anode containing a hydride alloy, and a thin film cathode containing NiO.sub.x active material. The battery is rechargeable and capable of fast discharging and recharging.
2. Description of the Related Art
Conventional aqueous Ni-MH batteries have at least three main disadvantages. Namely, such batteries deliver a relatively low amount of energy, such as 60 Wh/Kg and 120 Wh/L at the cell level, they produce oxygen gas during charge, and they have poor design flexibility.
Conventional Ni-MH batteries employ an alkaline liquid electrolyte to carry protons between very thick anodes and cathodes, which induces a valency change in the nickel, and a subsequent chemical change releases energy. The cell reaction for a conventional Ni-MH battery upon discharge is: EQU NiOOH+MH.fwdarw.Ni(OH).sub.2 +M
Since aqueous electrolytes are used in conventional Ni-MH batteries, generation of oxygen and hydrogen accompanies the valency change in Ni, according to the following reaction: EQU H.sub.2 O+e.fwdarw.1/2 H.sub.2 +OH.sup.- EQU 4 OH.sup.- .fwdarw.2H.sub.2 O+O.sub.2 +4e
The generated oxygen and hydrogen can be in gaseous form, thus constraining the design of the battery. For example, vessel design, seals and safety vents, as well as material selection are aspects that must be considered in order to accommodate the generated gases, which can lead to an increase in the mass of the battery. Accordingly, conventional alkaline Ni-MH batteries only deliver a fraction of the theoretical energy based on the mass of the components of the battery, because of the need to accommodate the gases that are generated.
While batteries using solid electrolytes have been constructed, as opposed to using aqueous electrolytes, such batteries suffer from a number of problems. For example, a Na/S/beta alumina electrolyte is known which is based on sodium ion conduction through aluminum oxide. However, batteries using such an electrolyte require a high operating temperature, e.g., 400.degree. C., and hence, have been abandoned as impractical.
Silver rubidium iodide and lithium/PbI batteries are are examples of other batteries, in which ion conduction occurs through solids. However, such batteries are not rechargeable.
Batteries employing lithium/polyethylene, oxide-LiCF.sub.3 SO.sub.3 /cobalt oxide and variations thereof, are also known. While such batteries have a specific energy of 130 Wh/Kg and neither contain liquids nor produce gas, they operate best at an elevated temperature, such as 60.degree. C., due to the polymer's poor conductivity at room temperature. Furthermore, the lithium ion conduction in such media is slower than that of protons, because a lithium ion is larger than a proton, and because of the formation of ion-pairs. Therefore, since ion conduction is rather slow, such batteries cannot be charged or discharged at high rates.
The Ni-MH solid state battery is a logical extension of the findings of Stefano Passerini, Bruno Scrosati and Vincent Hermann which were published in the Journal of Electrochemical Society Vol. 141, pp. 1025-28, April 1994 page 1025 regarding proton conduction and subsequent charge transfer reaction in an electrochromic device, rather than in a battery, containing a NiO.sub.x cathode, proton conductor and WO.sub.3 anode. Further, Masud Aktar refers to a solid state Ni-MH battery in U.S. Pat. No. 5,320,716, dated Jun. 14, 1994, at col. 12, which uses a hydride forming alloy, a proton conductor and Ni(OH).sub.2 cathode. However, Aktar's invention requires hydrogen gas for use as a proton reservoir in the battery which creates problems in hermetically sealing such a battery.